South Korean officials met with President Trump tonight, and they passed along an invitation from North Korean leader Kim Jong Un. President Trump says he will accept the invitation and will meet with North Korea by May to discuss denuclearization.

We're joined now by Frank Aum. He was a senior adviser on North Korea at the Defense Department, and he's now with the U.S. Institute of Peace. Welcome.

FRANK AUM: Thanks, Ari.

SHAPIRO: Do you think denuclearization is plausible? Or is a freeze the best-case scenario here?

AUM: It's hard to say. I think obviously denuclearization is our - the U.S. goal. It's hard to see North Korea doing that when their nuclear weapons are the primary guarantee of their security. But I think anything can happen when you have a high-level summit.

SHAPIRO: We've often heard that Kim Jong Un looks to Libya as a model, where we saw Moammar Gadhafi give up his nuclear weapons program and then he was overthrown. If that is the model that North Korea looks to, why would he consider giving up his program?

AUM: Well, it's not clear that Kim Jong Un will give up his program. I think we don't know that at this point. All we know is that he's willing to meet. And I think they certainly want to have this propaganda victory, the prestige of meeting with the U.S. president. So that in itself is a victory. But again, we don't know that he is willing to give up his nuclear weapons.

SHAPIRO: No sitting American president has met a North Korean leader before. How prepared do you think the Trump administration is for these talks?

AUM: Well, typically negotiations work best when we work from the bottom up - so using many confidence-building measures, having many meetings that culminate in a final agreement that's signed by the heads of state. The problem is that this is a long process, and it allows for many opportunities to fall apart. I think that the one positive of going big like this is that North Korea has a tendency and a preference to prefer big agreements, working through summits. They're a top-down regime. Their lower-level officials don't have the authority to negotiate.

Remember; in 1994, it took a meeting between Jimmy Carter and Kim Il Sung to lay the foundation for the agreed framework, and then later on lower-level officials hammered out all the details. So I think if we're going to hope for something big, it's better to do it at the highest levels.

That being said, there's a lot of pitfalls along the way. It takes - it's going to take a lot of time to talk about what the agenda is, where the summit will take place. So I think it was appropriate that the White House walked back the South Korean official statement and said that President Trump would meet with Kim Jong Un at a time and place to be determined rather than the end of May.

SHAPIRO: Well, you mentioned those talks in the 1990s. One big difference between now and then is that North Korea's nuclear program is far more advanced - some say 90, 95 percent of the way there. How does that change the dynamic here?

AUM: Right. So now North Korea has a lot more leverage. They have more things on the table to pull back from. And so the price will increase for the United States. It's unclear what North Korea is going to ask for, but I can guarantee that it's going to be a lot. And it's unclear whether the U.S. is willing to pay that price.

SHAPIRO: Now, President Trump has said sanctions will remain until an agreement is reached. The pressure will stay on. Is that a tenable position? Or do you think that the North Koreans will demand some concessions in exchange for engaging in these talks?

AUM: Well, I think that's exactly the right approach to take. We need to maintain the pressure throughout the process. But I do imagine that North Korea will ask for some sort of sanctions relief early on in addition to other asks as we move along in this process.

SHAPIRO: And so do you think that the Trump position is not a final one, that in fact there may be movement on sanctions?

AUM: Potentially, but I think North Korea would also be - will need to give up - in the White House terms it would be that North Korea would need to take concrete steps towards credible and verifiable denuclearization. So it'll certainly be more than just the freeze. It may be allowing the IAEA inspectors back into North Korea to conduct invasive monitoring and verification, and then possibly even more.

SHAPIRO: Yeah, I was going to ask you about those two words credible and verifiable. When we were talking about the Iran nuclear talks, there was a lot of discussion about how you know that they're keeping their promises. North Korea is one of the most secretive countries in the world. Could you envision a scenario where they allow international monitors in to confirm that they're doing what they say they're doing?

AUM: Well, North Korea has allowed international monitors into North Korea in the past. So that's definitely doable. The question is, will they have access to all the right sites and facilities? And then also, are there other covert sites and facilities, underground facilities, that North Korea is not declaring or revealing to the U.S.?

SHAPIRO: And when there's a trust deficit on both sides and North Korea says, yes, we're showing you everything we have and the U.S. doesn't believe it, what do you do with that?

AUM: Well, that's one of the stumbling blocks. And that's happened before in previous negotiations. Even in 2008 at the end of the six-party talks North Korea was dragging its feet in terms of providing sort of a verification process for its program. So that's certainly a concern and one we have to be mindful of in the process.

SHAPIRO: When you look at the American cast of characters here, I know you were former Defense Department, not State Department, but State will have a lot to do with this. And the special representative for North Korea has just retired. The assistant secretary of state for East Asia is going through her confirmation process. The Trump administration has not nominated anyone to be the South Korean ambassador. Does it make it much more difficult to go into a negotiating process like this when you don't have people in those roles?

AUM: Sure. It's - that's not helpful. I think it would be much more - the U.S. would be much more prepared if they had the right people in place. That being said, you know, there's a lot of talented and competent career foreign service officers at the State Department, and of course longtime Korea experts at the NSC. So I'm sure the Trump administration will make do, but I think at the same time it would be more reassuring if we had an ambassador and a special envoy to work on North Korea issues.

SHAPIRO: What advice would you give the negotiators going into this process?

AUM: To be prepared for anything, to - for the unexpected, and know that North Korea's very crafty. And it's going to be a long process.

SHAPIRO: What do you mean by the unexpected? What do you mean by saying they're crafty? I mean, are you talking about, like, some kind of tricks?

AUM: Well, I think we don't know what to expect from North Korea. We didn't know that they would agree to the inter-Korean summit with the South Korean president. We didn't know that they would extend an invitation to President Trump to meet. And so we just don't know. And also, Kim Jong Un is not Kim Jong Il. He's not his father. He's not his grandfather. Their motivations may be different. Their leverage is different. So again, we just need to be - need to expect anything at this point.

SHAPIRO: When you heard about this, did it make you optimistic?

AUM: Absolutely. I think of course maybe optimistic and also terrified at the same time. But I think...

SHAPIRO: (Laughter).

AUM: ...Any time there's news about high-level engagement between the U.S. and North Korea that's encouraging, especially in light of last year's provocations and sharp rhetoric and all the talk about bloody nose strikes.

SHAPIRO: Just in our last few seconds, why would announcement of talks make you more terrified?

AUM: Again, like I mentioned before, it requires a lot of preparation. And we don't know what President Trump's track record is in negotiations at least in the diplomatic space. So we don't have a lot to go on here.

SHAPIRO: Well, Frank Aum, thank you very much for joining us this evening.

AUM: Thank you.

SHAPIRO: Frank Aum was a senior adviser on North Korea at the Defense Department. He's now an expert at the U.S. Institute of Peace.